The present invention relates to a multi-focal spectacle lens, wherein dioptric power varies progressively between different zones of vision.
Such lenses are referred to generally as progressive lenses, and that surface of the spectacle lens which effects the progressively varying course of the dioptric power of the lens is referred to as the progressive surface. Copending U.S. application Ser. No. 570,589, filed Jan. 13, 1984 is generally descriptive of design principles for a progressive lens.
Progressive lenses are used to compensate for impaired vision of presbyopes (persons whose vision has been impaired by age), in that the power of accommodation of the eye has decreased. To date, progressive lenses for presbyopes have had an upper distance-vision region and a lower near-vision region, and these regions have been connected by a progressive zone. Within said zone, the dioptric powers of the upper and lower regions pass progressively and continuously into each other. Such a progressive lens is made in a single piece; it does not contain any disturbing separation line between the two viewing regions, and it is therefore esthetically very satisfactory. It is also very comfortable for the person wearing the spectacles, since the two viewing regions pass continuously into each other, without any jump in the viewed image, so that the transition from far vision to near vision occurs very naturally.
There are tasks which present difficulties for a person wearing spectacle lenses having two viewing-distance regions. Thus, for example, in the case of a computer-screen work station, the screen is at a viewing distance of 50 to 75 cm while the keyboard is at a viewing distance of 40 to 50 cm, and documents may also have to be viewed at positions alongside the screen and/or alongside the keyboard. In principle, it would be possible in such case to work with spectacle lenses which have only the two regions which correspond to said viewing distances. But certainly, the user would then not clearly see objects or persons away from his work station; i.e., he would have to remove his glasses to look a distance away or, if his vision is impaired, to change them.
To avoid this serious disadvantage, a paper in the journal "Optometrie" 5 (1984), 208/213, proposes special multiple-power lenses having three viewing-distance regions, in the manner of the known trifocal lens. A separation line delimits these regions from each other, and the separation lines are clearly visible; thus, each change in glance from distance vision to the region of the screen, or from the screen region to the region of the keyboard, entails a jump in the image.
The condition is similar in the case of spectacle lenses known from West German Pat. No. 3,127,148 (corresponding to U.S. Pat. No. 4,484,804). Such lenses can have three spherical regions of different dioptric value and all of these regions have a common tangent in the plane of the vertical meridian, i.e., they adjoin each other without seam. However, at lateral offset from this meridian, the viewing regions are connected by a transition surface, wherein seam lines form edges and thus points of optical jump due to the local difference in the involved radii of curvature. It can readily be seen that such a spectacle lens is pervaded by edges and can thus in no way be satisfactory. Furthermore, such a lens cannot be considered a progressive lens since it has jumps in curvature and thus in the action, even along the meridian.
From U.S. Pat. No. 2,878,721, it is known to develop a progressive lens having an extremely long progressive zone. In this way, regions lateral of this zone can be shaped such that they are relatively low in error, over a limited lateral extent. But the vertical distance between distance effect and near effect is too great in the case of such a lens. There is the additional difficulty that such a lens has no regions which are associated with specific fixed viewing distances, so that the user must in every viewing situation incline his head to seek the suitable viewing orientation.